Atmospheric pressure non-thermal plasma device to clean and sterilize the surfaces of probes, cannulas, pin tools, pipettes and spray heads

ABSTRACT

The present invention relates to methods and apparatuses for the use of atmospheric pressure non-thermal plasma to clean and sterilize the surfaces of liquid handling devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 10/858,272, filed Jun. 1, 2004 entitled“Atmospheric Pressure Non-Thermal Plasma Device to Clean and SterilizeThe Surfaces of Probes, Cannulas, Pin Tools, Pipettes and Spray Heads”,which application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/478,418, entitled “Atmospheric PressureNon-Thermal Plasma Device to Clean and Sterilize The Surfaces of Probes,Cannulas, Pin Tools, Pipettes and Spray Heads”, filed on Jun. 16, 2003,both prior applications of which are herein incorporated by reference intheir entirety.

BACKGROUND OF THE INVENTION

Within the disciplines of the clinical, industrial and life sciencelaboratory, scientists perform methods and protocols with extremelysmall quantities of fluids. These fluids consist of many categories andtypes with various physical properties. Frequently, volumes are workedwith that are between a drop (about 25 microliters) and a fewnanoliters. There are a number of standard methods employed to transferliquid compounds from a source by aspirating the liquid from such fluidholding devices into a fluid handling device having a probe, cannula,pin tool or other similar component or plurality of components thatmove, manually or robotically, and then dispensing, from the same probeor plurality of probes, into another fluid holding device.

Four common techniques are (1) a scheme using a probe or cannula, thatmay or may not be coated with a layer of material or special coating,which is attached directly or by a tube to a pumping device, (2) ascheme using a disposable pipette instead of the probe/cannula butotherwise similar, (3) a scheme using a spray head with one or aplurality of openings and pumping system that physically propelsmultiple precisely metered microdroplets, and (4) a scheme using metalshafts with precisely machined hollowed out spaces at their ends thathold the fluid by surface tension (commonly referred to as a “pintool”).

As routine a process as fluid transfer is in the laboratory, technicalchallenges remain to achieve suitable levels of cleanliness of thedispensing devices. Currently the fluid handling devices undergo a “tipwash” process wherein they are cleansed in between use with a liquidsolvent, such as DMSO or water. After the “tip wash” process, the usedand now contaminated liquids must then be properly disposed of withrespect to the required environmental regulations. As an alternative tothis wet “tip wash” process, atmospheric pressure plasma can be used toreplace the liquid cleaning process with a “dry” plasma cleaningprocess, thus eliminating the need for the handling and disposal ofsolvents that are biohazards and environmentally unfriendly.

The term “plasma” is generally used to denote the region in an electricgas discharge that has an equal number of positive ions and negativeelectrons (N. St. J. Braithwaite, “Introduction to gas discharges”Plasma Sources Science and Technology, V9, 2000, p517-527; H. Conrads etal., “Plasma Generation and Plasma Sources” Plasma Sources Science andTechnology, V9, 2000, p441-454). A non-thermal, or non-equilibrium,plasma is one in which the temperature of the plasma electrons is higherthan the temperature of the ionic and neutral species. Within anatmospheric pressure non-thermal plasma, there is typically an abundanceof other energetic and reactive particles, such as ultraviolet photons,excited and/or metastable atoms and molecules, and free radicals. Forexample, within an air plasma, there are excited and metastable speciesof N₂, N, O₂, O, free radicals such as OH, NO, O, and O₃, andultraviolet photons ranging in wavelengths from 200 to 400 nanometersresulting from N₂, NO, and OH emissions.

The “dry” plasma cleaning process is achieved by exposing the surfacesof the fluid handling devices or other components to the atmosphericpressure plasma. The above mentioned reactive and energetic componentscan now interact with any contaminants on the surfaces, therebyvolatizing, dissociating, and reacting with the contaminants, to formsmaller and benign gaseous compounds that are vented off through theplasma cleaning device.

In addition to removing various unwanted compounds, the plasma can alsobe used to sterilize the surfaces of the fluid handling devices. Thesame ultraviolet photons, especially those with wavelengths below 300nm, the free radicals and metastable molecules, and the plasma electronsand ions, provide a very harsh environment in which bacteria, viruses,fungi and their corresponding spores are lysed or otherwise renderednon-viable and either partially or completely volatized into gaseouscompounds.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, there is provided anapparatus for cleaning a fluid handling device. In one embodiment, theapparatus includes an array of channels, each made of a dielectricmaterial and configured to accommodate a single fluid handling device,at least one electrode in contact with each channel for producing adiscrete plasma in each channel, and at least one conducting groundadjacent to the array of channels. In one embodiment of the presentinvention, there is provided an apparatus that has at least oneconducting ground adjacent to each of the channels. In anotherembodiment of the present invention, a fluid handling device is theconducting ground. In yet another embodiment, a fluid handling deviceforms a conducting ground.

In an embodiment of the present invention, a plasma is produced in aplasma cleaning apparatus by applying a voltage in the range from about5000 Volts to 15000 Volts.

In an embodiment of the present invention, a channel of a plasmacleaning apparatus is cylindrical. In another embodiment, a channel of aplasma cleaning apparatus is rectangular. In one embodiment of thepresent invention, a channel of a plasma cleaning apparatus is closed onone end. In another embodiment, a channel of a plasma cleaning apparatusis open on both ends.

In one embodiment of the present invention, there is provided a plasmacleaning apparatus that is in direct communication with a vacuum source.

In an embodiment of the present invention, an apparatus may contain anarray of plasma cleaning apparatuses. In one embodiment, an array ofplasma cleaning apparatuses is in an arrangement corresponding to amicrotiter plate format.

In one embodiment of the present invention, there is provided a plasmacleaning apparatus containing at least one rare gas.

In an embodiment of the present invention, there is provided anapparatus for cleaning a fluid handling device, wherein the apparatuscontains an array of channels in a configuration corresponding to amicrotiter plate. In one embodiment, each channel includes a dielectricmaterial and is configured to accommodate a single fluid handlingdevice. In one aspect, there is at least one electrode in contact witheach channel for producing a discrete plasma in each channel and,additionally, there is a continuous conducting ground adjacent to thearray of channels. In one embodiment, the channels of an apparatus ofthe invention are cylindrical. In another embodiment, the channels of anapparatus of the invention are rectangular.

Another embodiment of the present invention features an apparatus forcleaning a fluid handling device, wherein the apparatus contains anarray of channels in a configuration corresponding to a microtiterplate, further wherein each channel consists of a dielectric materialand is configured to accommodate a single fluid handling device. In oneembodiment, there is at least one electrode in contact with each channelfor producing a discrete plasma in each channel and additionally, thereis a conducting ground adjacent to each channel. In one embodiment, afluid handing device forms the conducting ground for the channel inwhich the device is accommodated. In one embodiment, the channels of anapparatus of the invention are cylindrical. In another embodiment, thechannels of an apparatus of the invention are rectangular.

In an embodiment of the present invention, a fluid handling device isinserted into a channel of a plasma cleaning apparatus such that the tipof the fluid handling device is located at about the center of theplasma field.

In one embodiment the present invention, there is provided a method ofcleaning a fluid handling device by positioning at least a portion of afluid handling device within the interior of a channel of a plasmacleaning apparatus and forming a plasma within the interior of eachchannel in order to clean the fluid handling device. In one embodimentof the present invention, there is provided a method of cleaning aplurality of fluid handling devices by positioning at least a portion ofeach of a plurality fluid handling devices within the interior of adiscrete channel of a plasma cleaning apparatus and forming a plasmawithin the interior of each of the discrete channels to clean theplurality of fluid handling devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutea part of this specification, illustrate embodiments of the presentinvention and, together with the general description given above and thedetailed description given below, serve to explain the features of thepresent invention. Some aspects of the drawings are not labeled, but areincluded to provide further details of the present invention. Further,in some drawings, if a feature is present more than once in a drawing,the feature may be referenced only once.

FIG. 1 is a cross section view of one embodiment of an atmosphericpressure plasma-based cleaning device of the present invention.

FIG. 2 is a top angle view of one embodiment of an atmospheric pressureplasma-based cleaning device of the present invention.

FIG. 3 is a cross section view of one embodiment of an atmosphericpressure plasma-based cleaning device of the present invention, whereinthe upper dielectric portion is extended perpendicularly outward.

FIG. 4 is a top angle view of one embodiment of an atmospheric pressureplasma-based cleaning device of the present invention, wherein the upperdielectric portion is extended perpendicularly outward.

FIG. 5 is a cross section view of one embodiment of an atmosphericpressure plasma-based cleaning device of the present invention, whereina conducting surface is situated adjacent to the top of aperpendicularly outward extended dielectric.

FIG. 6 is a top angle view of one embodiment of an atmospheric pressureplasma-based cleaning device of the present invention, wherein aconducting surface is situated adjacent to the top of a perpendicularlyoutward extended dielectric.

DETAILED DESCRIPTION OF THE INVENTION

In certain embodiments of the design of an atmospheric pressure plasmadevice according to the present invention, a dielectric barrierdischarge (also known as a “silent discharge”) scheme is used, where atleast one electrode to which an alternating voltage is applied, includesan insulating dielectric (U. Kogelschatz et al. “Dielectric-BarrierDischarges, Principles and Applications” J. Phys IV France, 7, 1997,C4-47). The electrodes may comprise any conductive material. In oneembodiment, a metal may be used. Metals useful in the present inventioninclude, but are not limited to, copper, silver, aluminum, andcombinations thereof. In another embodiment of the present invention, analloy of metals may be used as the electrode. Alloys useful in thepresent invention include, but are not limited to stainless steel,brass, and bronze. In another embodiment of the present invention, aconductive compound may be used. Conductive compounds useful in thepresent invention include, but are not limited to indium-tin-oxide.

In one embodiment, an electrode may be formed using any method known inthe art. In an embodiment, an electrode may be formed using a foil. Inanother embodiment, an electrode may be formed using a wire. In yetanother embodiment, an electrode may be formed using a solid block. Inanother embodiment, an electrode may be deposited as a layer directlyonto the dielectric. In one embodiment, an electrode may be formed usinga conductive paint.

In an embodiment of the present invention, a plasma is obtained in adielectric barrier discharge (DBD) when, during one phase of the appliedalternating voltage, charges accumulate between the dielectric surfaceand the opposing electrode until the electric field is sufficiently highenough to initiate an electrical discharge through the gas gap (alsoknown as “gas breakdown”). During an electrical discharge, an electricfield from the redistributed charge densities may oppose the appliedelectric field and the discharge is terminated. In one embodiment, theapplied voltage-discharge termination process may be repeated at ahigher voltage portion of the same phase of the applied alternatingvoltage or during the next phase of the applied alternating voltage.

In another embodiment of the present invention, a corona dischargescheme may be used (E. M. van Veldhuizen, W. R. Rutgers. “CoronaDischarges: fundamentals and diagnostics” Invited Paper, Proceedings ofFrontiers in Low Temperature Plasma Diagnostics IV, Rolduc, TheNetherlands, March 2001, pp. 40-49). In one embodiment, a coronadischarge scheme may use asymmetric electrodes. In one embodiment of thepresent invention, a discharge develops within a high electric fieldregion near the area of strongest curvature of a sharp electrode. If theapplied voltage or electrode gap distance is such that the dischargecannot transverse the gas gap, then the resulting corona discharge willbe limited by electron recombination and space charge diffusion. In oneembodiment of the present invention, the tip of a probe, cannula or pintool can serve as the region of strongest curvature and resulting highelectric field to initiate a corona discharge.

Depending on the geometry and gas used for the plasma device, theapplied voltages can range from 500 to 20,000 peak Volts, withfrequencies ranging from line frequencies of 50 Hertz up to 20Megahertz. In an embodiment of the present invention, the frequency of apower source may range from 50 Hertz up to 20 Megahertz. In anotherembodiment, the voltage and frequency may range from 5,000 to 15,000peak Volts and 50 Hertz to 50,000 Hertz, respectively. By way of anon-limiting example, such parameters of voltage and frequency arecommonly found in neon sign ballasts for lighting purposes (UniversalLighting Technologies, Inc, Nashville, Tenn.).

Dielectric materials useful in embodiments of the present inventioninclude, but are not limited to, ceramic, glass, plastic, polymer epoxy,or a composite of one or more such materials, such as fiberglass or aceramic filled resin (Cotronics Corp., Wetherill Park, Australia). Inone embodiment, a ceramic dielectric is alumina. In another embodiment,a ceramic dielectric is a machinable glass ceramic (CorningIncorporated, Corning, N.Y.). In one embodiment, a glass dielectric is aborosilicate glass (Corning Incorporated, Corning, N.Y.). In anotherembodiment, a glass dielectric is quartz (GE Quartz, Inc., Willoughby,Ohio). In one embodiment, a plastic dielectric is polymethylmethacrylate (PLEXIGLASS and LUCITE, Dupont, Inc., Wilmington, Del.). Inyet another embodiment, a plastic dielectric is polycarbonate (Dupont,Inc., Wilmington, Del.). In still another embodiment, a plasticdielectric is a fluoropolymer (Dupont, Inc., Wilmington, Del.).Dielectric materials useful in embodiments of the present inventiontypically have dielectric constants ranging between 2 and 30.

The gas used in a plasma device of embodiments of the invention can beambient air, pure oxygen, any one of the rare gases, or a combination ofeach such as a mix of air or oxygen with argon and/or helium. Also, anadditive can be added to the gas, such as hydrogen peroxide, to enhancespecific plasma cleaning properties.

FIG. 1 shows a cross section of an embodiment of a DBD plasma cleaningdevice. In one embodiment, a dielectric includes a hollow open endeddielectric channel 101, with a thickness W from about 0.5 mm to about 3mm and a length L from about 1 cm to about 5 cm. Coupled to the outsideof the dielectric is an electrode 102, with an arbitrary thickness and alength I of about 0.5 to about 4 cm, which is connected to an AC powersupply 104. The exact dimensions of dielectric channel 101 are dependenton the properties of the materials used for fabrication. In anembodiment of the present invention, the dielectric constant anddielectric strength of a material may allow larger or smaller lengthsand/or thicknesses of such materials.

In one embodiment, a plasma cleaning device is cylindrical. In anotherembodiment of the present invention, a plasma cleaning device isrectangular. In yet another embodiment, a plasma cleaning device of thepresent invention is elliptical. In still another embodiment, a plasmacleaning device of the invention is polygonal. Referring to FIG. 1, inone embodiment of the present invention, the end of a grounded fluidhandling device 103 is inserted into the dielectric channel to a pointin between electrode 102 at the midpoint of length I of electrode 102,and acts as the opposing electrode. Plasma is thereby formed in betweenthe outer surface of the fluid handling device 103 and the inner wallsof the dielectric channel 101. In one embodiment, a plasma is adielectric barrier discharge plasma. In another embodiment, a plasma isa corona discharge plasma. The free space H between the top and bottomedges of electrode 102 and the top and bottom edges of dielectricchannel 101 is spaced a sufficient distance to prevent arcing betweenelectrode 101 and fluid handling device 103, which in this embodimentacts as a ground. In one embodiment, the space is about 0.5 mm to about10 mm to prevent arcing around the dielectric. In one embodiment, theminimum dimensions of space H may be determined as the distance requiredsuch that the corresponding electric field circumventing dielectric 101,but between electrodes 103 and 102, is not sufficient to induce a gasbreakdown directly between 103 and 102. It will also be understood thatthe maximum dimension of space H may be determined by how far the tip offluid handling device 103 can be inserted into the channel formed bydielectric 101.

Any volatized contaminants and other products from the plasma may bevented through the bottom of the device by coupling the bottom of thechamber formed by the dielectric to a region of negative pressure. Inone embodiment, a region of negative pressure is a vacuum. In oneembodiment, a vacuum is in direct communication with a channel of theplasma device and is used to draw plasma products through the bottom ofa plasma device.

FIG. 2 shows an embodiment of a DBD plasma cleaning device with aplurality of dielectric barrier discharge structures, with eachindividual plasma unit similar to that shown in FIG. 1. Outer surface203 of the individual dielectric channels 201 are all coupled to acommon outer electrode 202. In one embodiment, electrode 202 isconnected to an AC power supply. In another embodiment, a power supplyis a DC power supply. In one embodiment, a DC power supply is pulsed andemploys a square waveform. In another embodiment, a DC power supply ispulsed and employs a sawtooth waveform.

A plurality of grounded fluid handling devices can be inserted in theplasma device and be simultaneously processed. The spacing between eachof the individual plasma devices within the plurality are determined bythe geometries of the fluid handling devices to be inserted. Typicalgeometries for dielectric structure 201 can follow those set by theSociety for Biomolecular Engineering, Microplate Standards DevelopmentCommittee for 96, 384, or 1536 well microplates (Publication ANSI/SBS4-2004, “Well Positions for Microplates,” January 2004, The Society forBiomolecular Screening, www.sbsonline.com). Other geometries includesingle opening units and openings in linear and two dimensional arrays.

Several procedures may be used to clean or sterilize the inner and outersurfaces of the fluid handling device. To clean, sterilize, or otherwiseprocess the inner surfaces, the reactive and energetic components of theplasma are repeatedly aspirated into the fluid handling device, usingthe fluid handling devices' aspirating and dispensing capability, withthe aspiration volume, rate, and frequency determined by the desiredamount of cleaning/sterilization required.

As shown in FIG. 1, in one embodiment of the present invention, forcleaning or sterilizing the outer surfaces of a fluid handling device,the end of fluid handling device 103 can be inserted to a positionbefore or at the top of electrode 102 to just clean the end of fluidhandling device 103, or it can be inserted to a position further belowthe top level of electrode 102 to clean the outer surfaces of the fluidhandling device. The period of time the plasma is on and the reactiveand energetic components are in contact with the surfaces is alsodetermined by required processing parameters.

In an embodiment of the present invention, the DBD plasma device mayhave its upper dielectric portion extended perpendicularly along Arrow Aso that powered electrode 302 is also covered from the top as shown inthe representative cross section in FIG. 3. This configuration allowsthe spacing J between electrode 302 and dielectric 301 to be smallerthan the spacing H for the preventing of arcing around dielectric 301.In an embodiment, the minimum dimensions of space J may be determined asthe distance required such that the corresponding electric fieldcircumventing dielectric 301, and between electrode 302 and electrode303, here the fluid handling device, is not sufficient to induce a gasbreakdown directly between 303 and 302. In one embodiment, the maximumdimension of space J may be determined by how far the tip of fluidhandling device 303 is inserted into a plasma cleaning device. In oneembodiment of the invention, the tip of a fluid handling device 303 issituated midway in a plasma field. In another embodiment, the tip of afluid handling device 303 is situated at about the center of a plasmafield within a plasma cleaning device. In one embodiment, the tip of aliquid handling device 303 is inserted into a plasma cleaning device tothe midpoint of electrode 302. In another embodiment, the tip of a fluidhandling device 303 is placed within the region of maximum plasmadensity. The thickness W of dielectric 301 is similar to that discussedelsewhere herein with respect to FIG. 1. Furthermore, there can be nospacing J, such that the top of electrode 302 is adjacent to the bottomof perpendicularly extended dielectric 301. This will result in a plasmabeing created when the grounded fluid handling device is brought near tothe top of dielectric 301 but still outside the dielectric channel.

FIG. 4 illustrates an embodiment of the present invention, including aplurality of DBD devices, each sharing a common extended upperdielectric 401, which covers common electrode 402 from the top.

In another embodiment, a conducting surface 503 of any thickness can beplaced adjacent to the top of the perpendicularly extended dielectric.FIG. 5 shows a cross section of one embodiment of a representativedesign with a hole in conducting surface 503 aligned with the opening indielectric surface 501. As shown in FIG. 5, inner edge M of conductingsurface 503 can vertically cover inner dielectric wall 504 of dielectric501 in addition to the top of the opening of dielectric 501. Ifconducting surface 503 is grounded, a plasma can now be formed inbetween the space K between the top of powered electrode 502 and inneredge M of grounded electrode 503. Referring to FIG. 5, in one embodimentof the present invention, the maximum distance of space K may bedetermined wherein the electric field between edge M of electrode 503located within the channel formed by dielectric 501 and inner dielectricwall 504 corresponding to the top of 502 is sufficient to allow for gasbreakdown and the formation of a plasma within the channel formed bydielectric 501.

In one embodiment of the present invention, the minimum distance ofspace K may be zero. In another embodiment of the present invention, theminimum distance of space K may be a value greater than zero. Theoptimization of space K facilitates the creation of a more uniform anddiffuse volumetric plasma inside the cylindrical channel formed bydielectric 501 when a grounded fluid handling device is inserted. In oneembodiment of the invention, K is a distance between zero mm and 20 mm.In one embodiment, K is a distance between 1 mm and 10 mm. In anembodiment, K is about 3 mm.

In one embodiment, conducting surface 503 can be left unconnected fromground by a switch so as to not have it participate as an electrodeduring the plasma cleaning/sterilization process. This will facilitatethe creation of a more concentrated plasma at the extreme end of thefluid handling device as opposed to a diffuse volumetric plasma aroundthe end.

FIG. 6 illustrates one embodiment of the present invention, featuring arepresentative design of a plurality of DBD plasma devices sharing acommon conducting surface 603, which can be grounded or ungrounded, anda common powered electrode 602, each separated by a commonperpendicularly extended dielectric 601.

In an embodiment of the present invention, a plurality of DBD plasmadevices are arranged in a format of a microtiter plate. Examples ofmicrotiter plate formats include, but are not limited to, a 96-wellplate format, a 384-well plate format, and 1536-well plate format.However, it will be understood that plate formats having fewer than 96wells, such as 48-well, 24-well, 12-well and 6-well formats, are alsouseful in embodiments of the present invention. In one embodiment, thephysical properties of a channel useful in embodiments of the presentinvention, such as a channel formed by a well in a microtiter plate, canbe determined based on the properties of the dielectric material used,the dimensions of such a channel, and the amount and character of energyused to produce a plasma within such a channel, as described in detailelsewhere herein. Similarly, the amount and character of energy used toproduce a plasma within a channel may be determined, as described indetail elsewhere herein, by analysis of the physical properties of sucha channel and the properties of the dielectric material used.

In an embodiment of the present invention, an array of liquid handlingdevices may also be in a format compatible with a microtiter plate. Inanother embodiment, an array of liquid handling devices compatible witha microtiter plate format may be cleaned using an apparatus or method ofthe present invention. Microtiter plate handling devices useful in thepresent invention include, but are not limited to those using an XYZformat for liquid handling, such as the TECAN GENESIS (Tecan, Durham,N.C.). Other microplate handling formats compatible with embodiments ofthe present invention include those used with instruments such as theBeckman Coulter FX (Beckman Coulter, Fullerton, Calif.) and the TekCelTekBench (TekCel, Hopkinton, Mass.).

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof. Thus, it isintended that embodiments of the present invention covers themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents. Further, eachand every reference disclosed herein is hereby incorporated by referencein its entirety.

1. A method of cleaning a fluid handling device, comprising: positioning at least a portion of the fluid handling device within an interior of a channel; and causing a plasma to be formed within the interior of the channel.
 2. A method of cleaning a plurality of fluid handling devices, comprising: positioning at least a portion of each of the fluid handling devices within an interior of a discrete channel of an array of channels; and causing a plasma to be formed within the interior of the discrete channels.
 3. The method of claim 2, further comprising venting volatized contaminants and other products from the plasma to a region of negative pressure.
 4. The method of claim 2, further comprising: providing a vacuum in communication with the channels; and drawing plasma products through the bottom of the array of channels.
 5. The method of claim 2, wherein the step of causing a plasma to be formed further comprises aspiring into the fluid handling device the reactive and energetic components of the plasma.
 6. The method of claim 2, wherein the step of positioning at least a portion of each of the fluid handling devices comprises inserting the end of each of the fluid handling devices into the channels a predetermined distance to clean the end of each fluid handling device.
 7. The method of claim 2, wherein the step of positioning at least a portion of each of the fluid handling devices comprises inserting the tip of each fluid handling device in a midway point in the plasma.
 8. The method of claim 2, wherein the step of positioning at least a portion of each of the fluid handling devices comprises inserting the tip of each fluid handling device in a region of maximum plasma density. 